The present invention relates to induction heating and, more particularly, to a method and apparatus for inductively heating workpieces having portions of different mass.
The present invention will be described herein in connection with a workpiece in the form of a connecting rod such as that used to connect the crankshaft and piston in an automotive engine and, in particular, in connection with the de-waxing and sintering of a connecting rod of powdered metal prior to final forming and sizing thereof. As will become apparent hereinafter, however, the invention is applicable to the heating of other metal workpieces having geometric configurations which provide for areas or portions of the workpiece to have different masses as well as to the de-waxing and sintering of such workpieces of compacted powdered metal other than connecting rods.
Heretofore, connecting rods of compacted powdered metal have been de-waxed and sintered prior to forging for final densification and sizing by staging a green, unsintered compact through a furnace, such as a rotary hearth furnace. The connecting rod is heated in a first zone of the furnace to a temperature of about 400.degree. F. to 800.degree. F., for example, to achieve migration and burn-off of the wax in the compact which is used to lubricate the particles during compacting. The connecting rod is then staged to a second heating zone in the furnace in which the temperature is elevated to a sintering temperature of about 2050.degree. F., after which the connecting rod is cooled to a temperature suitable for forging which takes place following discharge of the workpiece from the exit end of the furnace. Disadvantages of this process include the time required to elevate the connecting rod to a uniform temperature in the range required for de-waxing, and to then elevate the connecting rod to a uniform temperature required for sintering. Another problem results from the inability to obtain good heat zone separation and temperature control in the two zones of the furnace, and this can result in an improper de-waxing operation and excessive amounts of oxygen in the powdered metal product.
In a modification of the foregoing process, to improve the de-waxing procedure, the connecting rod is staged through a furnace in which the connecting rod is de-waxed and pre-sintered to a temperature of about 1600.degree. F., and the connecting rod is then cooled to room temperature. Thereafter, the connecting rod is reheated in a second furnace to the final sintering temperature of about 2050.degree. F. While this approach apparently improves the end product, the time required to heat the connecting rod in each furnace to the uniform temperature required for de-waxing, pre-sintering and sintering is excessive, and the total time required to sinter the product is further increased by the cooling down stage between the pre-sintering and final sintering operation.
In connection with the preheating and sintering and the preheating, pre-sintering and reheating procedures referred to above, as well as any heating of a metal workpiece of odd geometry providing areas of different mass, it will be appreciated that the time required to heat the workpiece to any given uniform temperature is directly related to the geometry of the workpiece. In this respect, the furnace, or zone of a multiple stage furnace, is maintained at a temperature to elevate the workpiece to the given temperature, and the portion or portions of the workpiece which are of the lowest mass will reach the furnace temperature sooner than the portion or portions of higher mass. Even though there is some distribution of heat from the lower mass area to the higher mass area or areas, it will be appreciated that it takes a longer period of time to heat the larger mass portions of the workpiece to the furnace temperature, especially if there is a considerable difference in mass between the areas of high and lower mass. Therefore, whether the workpiece is a metal workpiece being heated to a temperature for further processing such as by forging or die shaping, or is a compacted powdered metal workpiece being de-waxed and sintered or being reheated in preparation for final sizing and densifying, the production rate with respect to heating a series of such workpieces for further processing is undesirably reduced by the time required to heat the areas of high and low mass to a given temperature.